The present invention relates to pressure swing adsorption processes and more particularly to such processes for recovering hydrogen at high recoveries and high purities as an unadsorbed product from a feed gas mixture having relatively low hydrogen concentration.
Pressure swing adsorption processes are well-known for the separation of gas mixtures that contain components with different adsorption characteristics. For example, hydrogen production via pressure swing adsorption is a multi-million dollar industry supplying high purity hydrogen for chemical producing industries, metals refining and other related industries. The hydrogen is typically separated from gas mixtures having high hydrogen concentration, for example from reformers with or without shift reactors.
Typically, gas mixtures containing low hydrogen concentrations are combusted as fuel to recover the heating value from the hydrogen.
There are applications where recovery of hydrogen from feed gas mixtures having low hydrogen concentration would be useful. For example, in a molten carbonate fuel cell, synthesis gas (a gas comprising carbon monoxide and hydrogen) may be passed to an anode compartment while an oxidizing gas, typically air, is passed to the cathode compartment. Exhaust gas from the anode contains unreacted fuel (carbon monoxide and hydrogen) as well as water and carbon dioxide. In order to improve the efficiency of the fuel cell, carbon monoxide and hydrogen can be separated from the carbon dioxide and water and the carbon monoxide and hydrogen can be recycled to the anode as disclosed in U.S. Pat. No. 4,532,192 and U.S. Pat. Publ. 2004/0229102. The carbon dioxide that was separated may be recycled to the cathode since carbon dioxide is required for molten carbonate fuel cells.
It may be desirable to recover pure hydrogen from the exhaust stream of the anode. In this case, the pure hydrogen may be used as fuel to a proton exchange membrane (PEM) fuel cell or could be stored for use in fuel cell vehicles. In this application, the composition of the anode exhaust gas, following the water gas shift reaction, may contain about 70% carbon dioxide, 25% hydrogen, and 5% water, with less than 1% carbon monoxide, nitrogen and methane.
A problem facing the fuel cell industry is to recover a high percentage of hydrogen from feed gas mixtures containing low concentrations of hydrogen. It would be desirable to obtain high purity hydrogen from feed gas mixtures containing low concentrations of hydrogen with high hydrogen recovery.
An option being considered for distributing hydrogen in the future hydrogen economy centers on the separation of dilute hydrogen from natural gas. Hydrogen is produced in low-cost centralized facilities, and is then injected into natural gas pipelines for subsequent transport to distributed refueling stations along the gas pipeline network. High purity H2 is separated from the H2/natural gas mixture at each refueling station. Practical issues limit the H2 content of this gas to less than about twenty percent, with the remaining components consisting of typical natural gas species (e.g., methane, N2, C2+ hydrocarbons).
Recovery of hydrogen from refinery fuel gas is another area of interest. Currently, many refineries are in need of hydrogen for various unit operations including hydrocracking, hydrodesulfurization and reforming. Recovering hydrogen from various low hydrogen concentration feed streams could be an attractive option to building a new hydrogen production plant if appropriate technology existed. Refinery fuel gas is a collection of light gases generated in any number of processing units in the refinery. A typical composition of refinery fuel gas is about 25% hydrogen, about 67% C1-C6 hydrocarbons, about 2% oxygen, about 5% nitrogen, about 1% carbon dioxide and ppm levels of hydrogen sulfide. An efficient process to recover a high purity hydrogen stream from such a feed gas with low hydrogen content would be desirable.
Prior art teaches that pressure swing adsorption is not suitable for recovering hydrogen from feed gas mixtures containing low hydrogen concentrations. For example, Lancelin et al., “Hydrogen Purification by Pressure Swing Adsorption,” presented at the Hydrogen Symposium of the AFTP, 26 Feb. 1976) show a plot of hydrogen recovery as a function of the hydrogen concentration in the feed gas mixture using pressure swing adsorption. The plot shows a hydrogen recovery of about 80% for 70% hydrogen in the feed gas mixture, decreasing to about 60% hydrogen recovery for 50% hydrogen in the feed gas mixture.